Since at least the time of St Clement of Alexandria (150–215), natural law has been a fundamental concept in Catholic accounts of private and public morality. But what is natural about this law (an ontological question) and how we understand it (the epistemological question) has been far from stable. In the twentieth century, a number of competing theories became fashionable, and disagreements among their proponents fuelled an academic growth industry. Underpinning every account of natural law is a presupposition about how faith and reason are related, and a presupposition about how nature and grace are related. Depending on how one construes these primary relationships, one can come up with different accounts of natural law all purporting to be Catholic. There are commonly three different ways of reading the relationship between the couplets ‘nature and grace’ and ‘faith and reason’. These are often described in academic short-hand as Neo-Thomist, Transcendental Thomist and Neo-Augustinian or Augustinian-Thomist.